1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, particularly having a MIS (Metal Insulator Semiconductor Field Effect Transistor) using a conductive film as the gate electrode thereof.
2. Description of the Related Art
As of now, the miniaturization of a device is pursued so as to enhance the performance of the MISFET thereof. In the transistor with a gate electrode made of a conventional polysilicon, the depletion of the gate electrode can not be neglected and causes some problems. In this point of view, recently, the gate electrode is made of a metallic material instead of the conventional polysilicon.
In the conventional polysilicon gate electrode structure (containing a polycide structure, a salicide structure and a poluymetal structure), the threshold voltage of the transistor is determined on the impurity concentrations of the channel region and the polysilicon film. In the metal gate structure, however, the threshold voltage is determined on the impurity concentration of the channel region and the work function of the gate electrode. Therefore, the metal gate is required to have the work functions suitable for the NMOS (N Channel Metal Oxide Semiconductor) and the PMOS (P Channel Metal Oxide Semiconductor). For example, it is desired that the work function of the metal gate is set to 4.8 eV or more for the PMOS and the work function of the metal gate is set to 4.3 eV or below for the NMOS.
In the case that two kinds of metal gates are employed, one of the two kinds of metal gates is applied for the NMOS and the other of the two kinds of metal gates is applied for the PMOS (refer to Reference 1). In this case, however, the one or the other of the two kinds of metal gates is required to be removed so that the surface of the gate insulating film is exposed to air, the etching solution or etching gas and thus, the reliability of the gate insulating film is remarkably deteriorated.
[Reference 1] JP-A 2002-329794 (KOKAI)